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79215 Feldspathic Granulitic Impactite 553.8 Grams 79215 Feldspathic Granulitic Impactite 553.8 grams Figure 1: Photo of 79215 showing both freshly broken surface and patina covered exterior. Cube is 1 cm for scale. NASA S73-17184. Introduction metamorphism can be calculated from mineral pairs Sample 79215 was found on the surface near Van Serg (800 to 950 deg C). Crater (Muehlberger et al. 1974). It’s orientation on the surface is known from photographs (see figure 224 79215 has been dated at ~ 3.9 ± 0.1 b.y., with a long in Wolfe et al. 1981) and from the distinct “soil line” exposure to cosmic rays (170 - 330 m.y.) It is an around the sample (figure 1). Light-colored rocks in oriented sample with a thick patina and numerous the bottom of Van Serg Crater may be related. micrometeorite craters. 79215 is a holocrystalline, feldspar-rich rock with a Petrography granoblastic texture that was formed by a high McGee et al. (1979) and Neal and Taylor (1993) temperature metamorphic process of unknown origin. summarized what is known about 79215. This large The sample is low in KREEP elements, but high in rock is a coherent granulitic impactite, characterized meteoritic siderophiles. It is apparently an annealed by a granoblastic matrix of equant plagioclase grains aggregate of relict anorthositic and troctolitic (An95; 0.1 mm) which meet at 120 deg triple junctions cumulates, with an added meteoritic componet. Within with smaller intersertal grains of olivine, pyroxene and 79215 are numerous “oxide complexes” with opaque minerals (Bickel et al. 1976; McGee et al. surrounding reaction corona. The temperature of 1978)(figure 2). Lunar Sample Compendium C Meyer 2008 Figure 3: Photo of thin section 79215,73. Scale ~ 1 x 2 cm. Figure 4: Photo of thin section 79215,55. Scale ~ 1 x 2 cm. In spite of the variation in mineral mode, the mineral Figure 2: Photomicrographs of thin section 79215,62 (field of view is 0.7 mm). Top is plane-polarized light, compositions are rather constant (equilibrated?). middle is crossed-polarized, and bottom is reflected. NASA S79-27264, 27265 and 27263 respectively. McGee et al. (1978) studied numerous reaction corona surronding small “opaque complexes” in the matrix. These complexes consist of core assemblages which The mineralogic mode was determined for 9 regions may include spinel, ilmenite, armalcolite, troilite, rutile in 79215 (see tables). McGee et al. (1978) determined and metallic iron which are surrounded by corona of that the rock is about 72% matrix and 28% relict lithic plagioclase and occasionally olivine. Chomite spinel clasts. This variation is also seen in the chemical data is the dominant constiuent of the oxide assemblages. of Blanchard et al. (1977). On average the sample is The plagioclase in the corona are slightly zoned about 80 % plagioclase, 10 % olivine and 8 % pyroxene. outward from An96-90. Lunar Sample Compendium C Meyer 2008 Aninplagioclase Di 79215 Hd Bickel et al. 1976 75 80 85 90 95 troctolites 90 mg-suite pyroxene low-Ca in En norites 79215 80 En Fs gabbro- 70 Fo Fa norites compiled by C Meyer Figure 5: Olivine and pyroxene composition of ferroan- 60 anorthosite 79215 as (from Bickel et al. 1976). 50 Figure 7: Pyroxene and plagioclase composition of 79215. Pyroxene: Bickel et al. (1976), McGee et al. (1978), Cushing et al. (1999) and Hudgins et al. (2008) all report pyroxene composition (figure 5). Olivine: The olivine in 79215 has a range in Figure 6: Ni and Co content of metal grains in 79215 composition (Fo ). (from Bickel et al. 1976). 72-86 Chromite: Spinel ranges in composition from chromite to ulvospinel (McGee et al. 1978). The temperature of metamorphism can be calculated from the composition of high and low-Ca pyroxene Ilmenite: McGee et al. found 6-10% MgO in ilmenite. (Cushing et al. 1999) and/or olivine-ilmenite pairs (Anderson and Lindsley 1979). Cushing et al. Metallic iron: The Ni and Co data for metal in 79215 determined the annealing temperature as 1082 deg C, originally reported by Bickel et al. (1976), were simply while Anderson and Lindsley calculate only 660 deg repeated by Ryder et al. (1980). C. McGee et al. (1978) calculated anealling temperatures from 800 to 950 deg C from several Apatite: Megacrysts of subhedral apatite (up to 1.5 mineral pairs. mm) are observed in many of the sections and are believed to have formed during annealing (McGee et Mineralogy al. 1979). Bickel et al. (1976) give a partial analysis. Plagioclase: Plagioclase is An92-96. Some large grains have “necklaces” of olivine. Armalcolite: McGee et al. determined the chemical composition of armalcolite. Mineralogical Mode of 79215 Bickel et al. 1976 Plagioclase: 89 % 71.3 86.2 80.6 83.4 77.8 83.8 78.2 85.5 High-Ca Pyx. 3.2 0.9 1.7 4.3 5.3 3.4 4.2 3.5 4.1 Low-Ca Pyx. 1.9 0.4 0.1 4.1 3.3 6.3 3.9 9.4 3.3 Olivine 4.5 23.7 10.1 10.5 7.2 10.3 7.1 8.6 6.2 Oxides 0.4 0.4 0.5 0.4 0.4 0.2 0.2 0.2 0.5 Metal and troilite 0.1 .01 0.2 0.1 0.1 0.04 0.05 0.1 0.4 Apatite: 1 3.3 1.2 0.01 0.3 1.9 0.8 0 0 Lunar Sample Compendium C Meyer 2008 Mineralogical Mode of 79215 McGee et al. 1978 Matrix Mineral Annorthosite TroctoliteWhole Clasts clasts clasts Rock 72% 5% 20% 3% 100% Plagioclase: 78.8 68.3 93.3 42.4 80 Olivine: 13.4 19.3 5.8 30.5 12.7 Orthopyroxene: 5 4.5 0.8 21 4.6 Clinopyroxene: 2.4 0.1 6.1 2 Apatite: 7.8 0.4 Spinel: 0.1 0.1 Ilmenite: 0.1 0.1 Metal: 0.03 0.02 Troilite: 0.07 0.05 1000 79215 100 sample/ chondrite 10 1 0.1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Figure 9: Ar/Ar plateau for 79215 from McGee et al. Figure 8: Normalized rare-earth-element diagram (1978). for 79215 (data from tables). Figure 10: Ar/Ar plateau diagram for 79215 (Hudkins et al. 2008). Summary of Age Data for 79215 Ar/Ar McGee et al 1978 4.03 ± 0.02 b.y. Oberli et al. 1979 3.91 Figure 11: Ar/Ar pleatau diagram for 79215 Hudgins et al. 2008 3.871 ± 0.040 compared with that of 78155 (Oberli et al. 1979). Chemistry The chemical composition of several pieces of 79215 is higher than for the individual splits that were is given by Blanhard et al. (1977) and Lindstrom and measured. Hudgins et al. (2008) and Higuchi and Lindstrom (1986). Fruchter et al. (1975) measured the Morgan (1975) also analysed the sample, finding high Th content of the whole sample (550 grams) by siderophiles throughout. The REE pattern is flat and radiation counting. Note that the Th for the bulk rock similar to that of other feldspathic granulites (figure 8). Lunar Sample Compendium C Meyer 2008 Table 1a. Chemical composition of 79215. reference Bickel 76 McGee78 Higuchi and Morgan 1975 Fruchter75 Hudgins 2008 weight SiO2 % 43.8 (c ) 43.4 (c ) TiO2 0.3 (c ) 0.13 (c ) Al2O3 27.7 (c ) 28.5 (c ) FeO 4.6 (c ) 4.3 (c ) MnO 0.06 (c ) 0.04 (c ) MgO 6.3 (c ) 6.3 (c ) CaO 15.9 (c ) 15.9 (c ) Na2O 0.5 (c ) 0.38 (c ) K2O 0.1 (c ) 0.09 (c ) 0.113 (b) P2O5 0.17 (c ) S % 0.02 (c ) sum Sc ppm 7.7 8 7.8 7.8 (d) V Cr Co 16.5 18.6 16.3 15.4 (d) Ni 221 255 16 (a) 122 125 132 107 (d) Cu Zn 1.6 2.3 6.6 (a) Ga Ge ppb 39 33 36 (a) As Se 34 176 424 (a) Rb 0.465 0.489 0.187 (a) Sr 140 166 139 144 (d) Y Zr 47 38 49 46 (d) Nb Mo Ru Rh Pd ppb Ag ppb 0.71 1.16 3.37 (a) Cd ppb 1.04 0.98 1.92 (a) In ppb Sn ppb Sb ppb 1.3 2.79 6.36 (a) Te ppb 1.9 17 30 (a) Cs ppm 0.004 0.005 0.005 (a) Ba 89 83 111 135 (d) La 3.25 3.01 3.15 3.51 (d) Ce 8 7.1 7.9 9.2 (d) Pr Nd 4.7 4.7 4.2 5.4 (d) Sm 1.46 1.31 1.34 1.72 (d) Eu 0.81 0.8 0.82 0.83 (d) Gd Tb 0.31 0.29 0.3 0.36 (d) Dy Ho Er Tm Yb 1.41 1.43 1.39 1.5 (d) Lu 0.198 0.209 0.197 0.209 (d) Hf 1.39 1.23 1.26 1.15 (d) Ta 0.2 0.24 0.12 0.14 (d) W ppb Re ppb 0.5 1.9 2.1 (a) Os ppb Ir ppb 6.95 21.3 28.8 (a) 6 6 6.6 6.6 (d) Pt ppb Au ppb 1.67 8.27 15.6 (a) 1.1 5 1.9 2.2 (d) Th ppm 0.88 (b) 1.05 0.64 0.77 1.64 (d) U ppm 0.043 0.19 0.56 (a) 0.03 (b) 0.39 0.21 0.27 0.72 (d) technique: (a) RNAA, (b) radiation counting, (c ) calculated, (d) INAA Lunar Sample Compendium C Meyer 2008 Table 1b.
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